1. Field of the Invention
The present invention relates to a method of manufacturing silica waveguide optical components and, more particularly, a method of manufacturing silica waveguide optical components in which peeling and/or cracking of coated films does not occur in forming a waveguide by laminating silica glass films using the sol-gel method.
2. Description of the Prior Art
In the field of optical communication, as the introduction of optical fibers to the communication system for prospective subscribers increases, silica waveguide optical components composed of materials having the same quality as those of the existing optical fibers are of great importance.
These silica waveguide optical components are generally manufactured as described below. First, the method of manufacturing an optical component having a slab waveguide will be described.
A silica glass film whose substantial ingredient is SiO.sub.2 and which has a specified film thickness and a specified refractive index is formed as a lower cladding layer on a substrate having a specified thickness which consisting of, for example, single crystal Si.
Next, a silica glass film, in which a specified quantity of, for example, TiO.sub.2 is doped in SiO.sub.2 and which has a refractive index higher than that of the lower cladding layer, is coated as a core slab layer on the lower cladding layer. Then, a silica glass film of the same material as that of the lower cladding layer, which has a specified thickness and a specified refractive index, is coated as an upper cladding layer on the core slab layer. Thus, a slab waveguide optical component is provided which is so constructed that a slab-shaped core layer for light propagation is sandwiched between upper and lower cladding layers.
For an optical component having buried channel waveguides, the aforesaid core slab layer is formed, and thereafter unnecessary portions of the core slab layer are etched off by employing the photolithography method with the reactive ion etching technique to form channel waveguide cores spreading on the lower cladding layer with a specified plane pattern. Then, the aforesaid channel waveguide cores are covered with a silica glass film as an upper cladding layer. Thus, an optical component with a buried channel waveguide pattern is manufactured.
In both cases, the coating of silica glass films on the substrate is an indispensable process in manufacturing the above-described silica waveguide optical components.
In this case, the available methods of manufacturing aforesaid silica glass films include the flame hydrolysis deposition method, the plasma CVD method, the electron-beam evaporation method, and the sol-gel method. Among these methods, the sol-gel method has attracted considerable attention because the silica glass films can be produced by simple apparatuses with this method.
The sol-gel method is generally applied as described below.
First, a precursor sol of silica such as Si(OC.sub.2 H.sub.5).sub.4, which is the precursor sol of the substantial ingredient compound, is diluted by a solvent such as ethanol to prepare a sol for cladding layers whose viscosity is controlled appropriately. When a core layer is formed, a sol for the core layer is prepared in which a specified amount of a precursor sol of titania such as Ti[OCH(CH.sub.3).sub.2 ].sub.4 is mixed into the aforesaid sol for the cladding layers.
Next, a coated layer of a desired thickness is formed on, for example, a single crystal Si substrate, applying the aforesaid sol for cladding layer, using the spin coating method or dipping method. In the coating process using the spin coating method, the solvent of the sol volatilizes to provide gelation partially. Also, using the dipping method, the entire of coated layer is gelled by heating the coated layer at a relatively low temperature to accelerate the volatilization of solvent.
Afterward, the coated layer is heated usually at a temperature of 1000.degree. C. or higher in an oxygen atmosphere to sinter the gel of the coated layer, thereby transparent silica glass film being provided.
After a coated layer is formed by applying the sol again on the silica glass film formed by the aforesaid operation, sintering is carried out again, so that the second layer of silica glass film is laminated on the aforesaid first layer of the silica glass film.
The operation consisting of the formation of coated layer and sintering of the layer is repeated the specified number of times, and finally, the lower cladding layer having a specified thickness is formed.
For the core layer and the upper cladding layer, the silica glass film for each layer having a desired thickness can be formed by repeating the above-described operation.
In the above-described sol-gel method, the maximum film thickness of silica glass film formed by one unit operation consisting of the coating of sol and the consolidation of the coated layer, is about 0.7 .mu.m. In general, however, by one unit operation, the upper limit of film thickness for pure SiO.sub.2 laminated film is about 0.2 .mu.m, whereas that for TiO.sub.2 doped SiO.sub.2 laminated film with TiO.sub.2 concentration of 6.25 mol % is about 0.24 .mu.m.
When a silica glass film of a total thickness of 1 .mu.m or more is to be formed, therefore, it is necessary to repeat more than ten times the unit operation consisting of the coating of sol and the sintering of the coated layer.
However, when the unit operation consisting of the coating of the sol and the sintering of the coated layer is repeated about ten times, the silica glass film formed by the sol-gel method generally poses problems frequently in that cracks are formed in the film and/or peeling of the coated film occur. As a result, the yield in manufacturing lowers significantly.
In the case where the aforesaid buried channel waveguide optical components are manufactured, if the sol-gel method is used in forming the upper cladding layer to bury the channel waveguide pattern formed on the lower cladding layer, a coated layer having a uniform thickness throughout the entire surface of the channel waveguide pattern cannot be formed because the surface to which sol for cladding layer is applied is not flat due to the protrusive channel waveguide pattern having been formed on the surface of the lower cladding layer. For example, a relatively large amount of sol is applied to a concave portion that is formed by the side walls of the protruding channel waveguide pattern and the surface of the lower cladding layer, by which the thickness of the coated layer as a whole becomes relatively thick at locations along the protrusive channel waveguide pattern.
Therefore, if heating for consolidation of the coated layer is performed, after the unit operation consisting of the coating of the sol and the sintering of the coated layer is repeated one to several times likewise, cracking and/or peeling of the coated layer occurs at the portion of the upper cladding layer along the channel waveguide pattern, by which the yield is significantly lowered likewise.